1. Technical Field
The present disclosure relates to orthopaedic prostheses, and more particularly, to femoral prosthetic components with enhanced tibia femoral articulation characteristics.
2. Description of the Related Art
Orthopaedic prostheses are commonly utilized to repair and/or replace damaged bone and tissue in the human body. For example, a knee prosthesis may be used to repair damaged or diseased articular surfaces of the femur and/or tibia to restore natural function. A knee prosthesis may include a femoral component which replaces the articular surfaces of one or both of the natural femoral condyles, and/or the natural femoral sulcus. The femoral component typically articulates with a tibial component which replaces the proximal articular surface of the tibia.
A goal of knee replacement procedures is to reproduce natural knee kinematics using the associated prosthetic components. More generally such procedures seek to achieve kinematic characteristics that promote favorable patient outcomes including minimized pain, short recovery time, minimized risk of joint subluxation, and a long service life of the prosthesis components.
For example, a prosthetic knee may accommodate or induce “rollback” of the lateral femoral condyle in deep flexion, thereby replicating the rollback phenomenon experienced by a natural knee. Femoral rollback results from the natural tendency of the tibia and femur to rotate relative to one another, about their longitudinal axes, as the knee progresses from extension to deep flexion. This rotation process is referred to as “external rotation” because the anterior surface of the femur rotates externally or away from the center of the patient. Corresponding internal rotation of the femur accompanies lateral femoral “roll-forward” as the knee is articulated back toward extension.
External rotation and femoral rollback are a function of differential congruency between the medial and lateral articular surfaces of the knee. In particular, a normal natural knee has a high congruence between the medial femoral condyle and the corresponding medial tibial articular surface, but a lower congruence between the lateral femoral condyle and tibial articular surface. This differential congruency cooperates with interacting forces exerted by the soft tissues of the knee joint, including the posterior cruciate ligament (PCL), anterior cruciate ligament (ACL), medial and lateral cruciate ligaments (MCL and LCL), and the associated leg muscles linking the knee ligaments to the tibia or femur.
Previous design efforts have focused on providing prosthetic components which facilitate and/or accommodate femoral rollback and external rotation of the knee, particularly at medium and deep levels of knee flexion.
U.S. Pat. No. 5,219,362 to Tuke et al. discloses a knee prosthesis which permits internal/external rotation. The Tuke prosthesis has an asymmetric femoral component 1 with a medial condyle 3 (FIG. 1) that is larger than the lateral condyle 4. The medial condyle 3 has a surface portion which is largely spherical and is shaped to be substantially congruent with a spherical concave depression 6, while the lateral condyle 4 forms an incongruent contact with a trough-shaped depression 7 in the tibial component 2. The lateral depression 7, shown in FIG. 4, forms an arc shaped curve in plan view, with the curve having as its center the center point 9 of medial depression 6. The trough-shaped lateral depression 7 permits relative rotation between the tibia and femur about the axis through center point 9.
International PCT Application No. PCT/GB99/03407 to Walker discloses a femoral prosthesis which interacts with a corresponding tibial component to promote posterior displacement of the femoral component during flexion, and anterior displacement during extension. The Walker prosthesis utilizes a gradually changing frontal profile of the femoral condylar bearing surfaces from the distal to the posterior region and a corresponding intercondylar hump to achieve divergence of contact points between the femoral and tibial components from anterior to posterior, as shown in FIG. 2. For example, at zero degrees flexion as shown in FIG. 3(a), the distance separating contact points 11L and 11M is indicated as dimension X. At 60° flexion as shown in FIG. 3(b), the contact points 12L and 12M are separated by a distance ‘y’ which is greater than distance ‘x’. At maximum flexion of 120° as shown in FIG. 3(c), the contact points 13L and 13M are still further separated by a distance Z. The divergence of the femoral condyles creates an intercondylar groove which becomes deeper at higher levels of flexion, and the intercondylar hump formed on the tibial surface has a correspondingly increasing height from anterior to posterior (FIGS. 4(a)-4(c). The rise of the hump is steepest approaching the posterior side, which has the effect of assisting a roll forward movement as the femoral component rolls down the hump during extension.